The present invention relates to a technique for reducing a cost and size for the control of transient power application in a discharge lamp lighting circuit.
There has been known a lighting circuit for a discharge lamp (a metal halide lamp) comprising a DC power circuit, a DC-AC converting circuit and a starting circuit (a so-called starter circuit). For example, in such a structure that a DCxe2x80x94DC converter is used for the DC power circuit and a full bridge type circuit (in which four semiconductor switching elements make two pairs respectively to alternately carry out ON/OFF control) and a driver circuit thereof are used for the DC-AC converting circuit, a voltage having a positive polarity (or a negative polarity) to be output from the DCxe2x80x94DC converter is converted into a rectangular wave-shaped voltage in the full bridge type circuit and the rectangular wave-shaped voltage is then supplied to the discharge lamp.
The starting property of the discharge lamp is to be enhanced in respect of safety in utilization for a light source for a vehicle, for example, and it is preferable that a luminous flux should be increased to have a luminous flux value in a steady state as quickly as possible. In particular, in the case in which the discharge lamp in a cold state is to be lighted up after a comparatively long time passes since last lighting (a so-called cold start), it is necessary to apply a power which is several times (approximately twice or three times) as high as a steady-state power. Moreover, in the case in which a long time does not pass after the last lighting and a comparatively warm discharge lamp is to be lighted up (a so-called hot start), it is preferable to apply the power which is almost slightly higher than the steady-state power. More specifically, if a power which is almost equal to the power in the case of the cold start is applied in the latter case, a deterioration or a reduction in a lifetime might be caused by a thermal stress applied to the discharge lamp. Moreover, there is a problem in that a bad effect is produced by excessive light-up (safety is deteriorated).
Accordingly, it is necessary to predict and detect an extent of the cooling condition of the discharge lamp or a time elapsed after light-out and to then apply a power corresponding to the state of the discharge lamp. Referring to the cooling condition of the discharge lamp and the detection of a light-out time, the following methods have been known.
(1) A method of utilizing a discharge time constant of a capacitor; and
(2) a method of storing a voltage applied to a discharge lamp in a steady state and comparing the stored voltage with a voltage applied when the discharge lamp is lighted up.
First of all, referring to the (1), a capacitor is charged to be brought into a full charge state while a lighting circuit is started to light up the discharge lamp, and the capacitor starts a discharge when the discharge lamp is lighted out in response to an instruction for stopping the discharge lamp. At the time of next starting, if the number of electric charges remaining in the capacitor is smaller, an elapsed time is longer. Therefore, if the terminal voltage of the capacitor is detected, the time elapsed from last light-out (a light-out time) can be known. According to such a method, if the terminal voltage of the capacitor is low, a power supplied to the discharge lamp is increased in the control of the application of a transient power.
Referring to the (2), a voltage applied to the discharge lamp when the discharge lamp is lighted up is compared with a predetermined storage value (for example, a value of a voltage applied to the discharge lamp in a last steady state or a mean value thereof), and furthermore, a change in the voltage after the light-up and the asymptotic condition of the voltage to have a voltage value in the steady state are monitored. Consequently, it is possible to predict the cooling condition of the discharge lamp or the light-out time. According to such a method, therefore, it is possible to obtain an advantage that a necessary power can be applied to promote light emission while predicting the state of the discharge lamp.
In the method described above, however, the following drawbacks might be caused.
In the method (1), first of all, the capacity of a capacitor is large (for example, approximately several tens xcexcF are required when a time of several tens seconds to a few minutes is detected for the light-out time), and a disadvantage is produced in respect of a cost and a size. In addition, there is a possibility that the influence of a leakage current of the capacitor itself might adversely affect the characteristic of time detection because a discharge is required with a micro current.
Moreover, a memory (a nonvolatile RAM) is required for holding the storage value of the method (2). Also when the lighting circuit is started or the discharge lamp is lighted out, stored information is to be held (therefore, a dedicated memory which does not carry out an initializing process is to be provided separately or a computer is to be used for a control configuration).
The invention has an object to reduce a cost and a size by carrying out control without using a capacitor or a storage device which has a large capacity when performing transient power control corresponding to the cooling condition of a discharge lamp and a light-out time.
In order to solve the problems described above, the invention provides a discharge lamp lighting circuit comprising a DCxe2x80x94DC converting circuit for receiving a DC input voltage and converting the DC input voltage into a desired DC voltage, a DC-AC converting circuit for converting an output voltage of the DCxe2x80x94DC converting circuit into an AC voltage and then supplying the AC voltage to a discharge lamp, a starting circuit for generating a starting pulse to be sent to the discharge lamp, and a control circuit for controlling a power to be applied to the discharge lamp, the discharge lamp lighting circuit having the following structure.
The control circuit carries out power control in a steady state of the discharge lamp in response to a detection signal for a voltage applied to the discharge lamp and a current flowing to the discharge lamp and carries out output control of the DCxe2x80x94DC converting circuit for controlling a power to be transiently applied to the discharge lamp before a transition to the power control.
The power to be transiently applied to the discharge lamp is set to be higher than the power in the steady state and light emission of the discharge lamp is promoted to cause a luminous flux of the discharge lamp to approximate to a luminous flux in a steady state in a short time.
An increment in the power to be transiently applied which exceeds the power in the steady state is specified by the control circuit corresponding to a voltage applied to the discharge lamp, a variation in the voltage and a time elapsed from a time that the discharge lamp is lighted up or a start of an operation of a lighting circuit.
According to the invention, therefore, it is necessary to utilize only detected information required for the power control of a discharge lamp when controlling a transient power to be applied to the discharge lamp, and furthermore, it is possible to omit a circuit or a device which is required for detecting a cooling condition after the light-out of the discharge lamp or an elapsed time from last light-out to current light-up.